In recent years, an optical transmission network configuration using a wavelength division multiplexing (WDM) technique has been advanced in accordance with the increase in communication traffic.
The optical transmission network using the WDM technique utilizes, for example, an optical add-drop multiplexer (OADM) capable of passing (through), inserting (adding), and splitting (dropping) signal light for each wavelength, in addition to a terminal apparatus or a relay node.
FIG. 1 illustrates an example of a configuration of an optical transmission network 100.
The optical transmission network 100 illustrated in FIG. 1 includes, for example, an optical transmission station (Tx) 200, a plurality of OADMs 400-1 to 400-5, and an optical reception station (Rx) 300. The OADMs 400-1 to 400-5 are simply referred to as an OADM 400 below when they are not discriminated.
In an example illustrated in FIG. 1, signal light transmitted from the Tx 200 is inserted to wavelength-division multiplexed light transmitted to an optical transmission path by the OADM 400-1 (OADM #1) and then passes through the OADM 400-2 (OADM #2) and the OADM 400-3 (OADM #3). Further, the passed light is split at the OADM 400-4 (OADM #4) and is received by the Rx 300.
The OADM 400 includes, for example, an arrayed waveguide grating (AWG), a multilayer film filter, a fiber bragg grating (FBG), a wavelength selective switch (WSS) or the like to pass, insert, and split the signal light for each wavelength.
FIG. 2 illustrates an example of a configuration of the OADM 400.
The OADM 400 illustrated in FIG. 2 includes, for example, an optical amplifier 401, an optical coupler 402, an optical coupler 403, a WSS 404, an optical coupler 405, an optical amplifier 406, and an optical channel monitor (OCM) 407. In addition, the OADM 400 includes, for example, a demultiplexer 408, an optical receiver (Rx) 409, a multiplexer 410, and an optical transmitter (Tx) 411.
The wavelength-division multiplexed light input to the OADM 400 from the optical transmission path is amplified by the optical amplifier 401 and then is split in power by the optical coupler 402.
One of the wavelength-division multiplexed light split by the optical coupler 402 is demultiplexed by the demultiplexer 408 for each wavelength and is received by the Rx 409.
The other of the wavelength-division multiplexed light split by the optical coupler 402 is input to a route toward the WSS 404. In addition, each signal light transmitted from the Tx 411 or the like is multiplexed by the multiplexer 410 and is also input to the WSS 404.
The WSS 404 selects either one of the signal light included in the wavelength-division multiplexed light transmitted through the optical transmission path and the signal light inserted from the multiplexer 410 for each wavelength. For example, when the OADM 400 performs passing through on a signal light with a certain wavelength, the WSS 404 selects the signal light with the corresponding wavelength included in the wavelength-division multiplexed light transmitted through the optical transmission path. Meanwhile, when the OADM 400 performs adding on a signal light with a certain wavelength, the WSS 404 selects the signal light inserted from the multiplexer 410.
The signal light selected by the WSS 404 is output to the optical amplifier 406 and is amplified by the optical amplifier 406 to be output to the optical transmission path.
The wavelength-division multiplexed light input to the WSS 404 is split in power by the optical coupler 403 to be input to the OCM 407, and, similarly, the wavelength-division multiplexed light output from the WSS 404 is split in power by the optical coupler 405 to be input to the OCM 407.
The OCM 407 monitors, for example, a power level of an input signal of the WSS 404 and a power level of an output signal of the WSS 404 for each channel (wavelength) and controls the amount of attenuation in the WSS 404 based on the monitor result such that the signal light becomes a target power level for each wavelength. The OCM 407 may control the amount of amplification in the optical amplifiers 401 and 406 based on the monitor result.
Meanwhile, as a monitor control method of the wavelength selective switch, for example, JP 2010-245993 A discloses a method of detecting a channel in which the deterioration in the power level exists by monitoring input light and output light.
As an example of a configuration of a wavelength selective switch (WSS), a WSS using a mirror array is known as illustrated in FIG. 3. The WSS has good characteristics for a transmission band, a loss, or a polarization dependency of signal light.
The WSS 404 illustrated in FIG. 3 includes, for example, an output port 430, a plurality of input ports 431-1 to 431-3, a collimator 432, a dispersion element 433, a lens 434, and mirror array 435. However, a configuration of the WSS 404 with three-input and one-output illustrated in FIG. 3 is illustrative and the number of ports is not limited thereto.
For example, an optical fiber or the like is used for the input ports 431-1 to 431-3. Each wavelength-division multiplexed light input from the input ports 431-1 to 431-3 is converted into a parallel light beam by the collimator 432 to be emitted to a space and is incident on the dispersion element 433. Each wavelength-division multiplexed light incident on the dispersion element 433, is demultiplexed (dispersed) into light each having the wavelength by the dispersion element 433. In the example illustrated in FIG. 3, an arrangement direction Y of each port is vertical to a dispersion direction X by the dispersion element 433.
Here, diffraction grating is generally used for the dispersion element 433. The diffraction grating is an optical element in which a number of parallel grooves are periodically carved on a glass substrate. By using a diffraction phenomenon, the diffraction grating is possible to emit light beams having plural wavelengths, which are incident at a constant angle, in different angles for each wavelength to thereby separate the light beams for each wavelength.
Since the light beams dispersed into each wavelength by the dispersion element 433 diffuses as they are, the light beams are converted into parallel light beams again by the lens 434.
Generally, an arrayed mirror having mirrors so called MEMS mirrors formed with a micro electro mechanical systems (MEMS) technique is used for the mirror array 435. For example, one MEMS mirror is arranged in the mirror array 435 in correspondence with light having one wavelength separated by the dispersion element 433.
The MEMS mirror has a configuration in which an inclination angle of a reflection surface is variable by an electromagnetic force and an output port to which reflected light is guided is determined in correspondence with the inclination angle of the reflection surface. The WSS 404 illustrated in FIG. 3 may control the amount of its attenuation by changing the angle of the MEMS mirror to thereby change optical coupling efficiency between the signal light and the output port.
FIG. 4 is a diagram of the WSS 404 illustrated in FIG. 3 when viewed in the X direction, and FIG. 5 is a diagram of the WSS 404 illustrated in FIG. 3 when viewed in the Y direction.
Incidentally, one of parameters indicating performance of the above-described wavelength selective switch is, for example, a transmission band characteristic.
As a ratio (W/ω) of a width W of the mirror to a beam diameter ω of the light collected onto the MEMS mirror corresponding to each wavelength is greater and a deviation of a center wavelength is smaller, a transmission band of the wavelength selective switch becomes wider. In other words, as the width W of the MEMS mirror is large, the beam diameter ω on the MEMS mirror is small, and a collecting position of the light corresponding to each wavelength of an International Telecommunication Union (ITU) grid coincides with the center of the MEMS mirror, the transmission band becomes wider. The ITU grid is a wavelength standardized by the International Telecommunication Union (ITU).
As the transmission band of the wavelength selective switch becomes wider, there are advantages that an upper limit of an available bit rate rises or that the number of multi-stage connections of the wavelength selective switches can increase. In other words, as the transmission band of the wavelength selective switch becomes narrower, it is difficult to ensure a good transmission characteristic.
Here, the MEMS mirror cannot be controlled at a target angle in some cases due to deterioration or arrangement deviation of the mirror array 435 inside the WSS 404 or an optical element such as the dispersion element 433, deviation of an refractive index due to change in composition of a filling gas, deterioration of a driving circuit, failure thereof or the like.
In such case, the signal light input to the WSS 404 may be unavailable to be output normally. For example, the signal light is not output from the WSS 404, or the transmission band of the WSS 404 is deteriorated.
Specifically, under the circumstances where a signal rate per one wavelength becomes a high speed such as 10 G, 40 G, 100 G, . . . , a signal light spectrum tends to be thicker in a wavelength direction, and thus the deterioration in the transmission band of the WSS 404 more easily causes degradation in signal quality.
Therefore, in some cases, an operation state of the wavelength selective switch is monitored based on a power level of main signal light input to the wavelength selective switch and a power level of the main signal light output from the wavelength selective switch.
However, a method of monitoring the input/output power levels of the main signal light is unavailable to monitor the deterioration in the transmission band of the wavelength selective switch. Further, since the main signal light is constantly needed at the time of the monitor control of the wavelength selective switch, the monitor control of the wavelength selective switch is unavailable before the optical transmission network is in service.
Meanwhile, although a monitor light may be input to the wavelength selective switch together with the main signal light, the interference with the main signal light may occur.
Further, even in the variable dispersion compensator capable of performing variable dispersion compensation processing on the input light, there is a problem similar to the above.